ROLE OF MUSCLES IN LUMBAR SPINE STABILITY IN MAXIMUM EXTENSION EFFORTS

Many problems of the lumbar spine that cause pain are attributed to in stability, The ligamentous spine (without muscles) is unstable at very low compressive loads. This study examined the hypothesis that instab ility of the lumbar spine is prevented under normal circumstances by t he stiffness of spinal musculature, without active responses from the neuromuscular control system. The effect of muscle activity (force and stiffness) on the stability of the lumbar spine was analyzed for maxi mum voluntary extension efforts with different spinal postures in the sagittal plane. The analysis included realistic three-dimensional repr esentation of the muscular anatomy with muscles crossing several motio n segments. The stiffness of motion segments was represented using in vitro measured properties. Under a range of conditions with maximum ex tension effort, active muscle stiffness was required to prevent the lu mbar spine from buckling. The dimensionless value of the muscle stiffn ess parameter q as a function of activation and length had to be great er than a critical value in the range of 3.7-4.7 in order to stabilize the spine. Experimentally determined values of q ranged from 0.5 to 4 2. These analyses demonstrate how changes in motion segment stiffness, muscle activation strategy, or muscle stiffness (due to degenerative changes, injuries, fatigue, and so on) might lead to spinal instabilit y and ''self-injury.''